The present invention relates generally to purification of metal, and particularly to a process and apparatus in which the purification is effected by both a fractional crystallization and an electrolytic process.
Of the known methods of purifying aluminum, two are fractional crystallization involving the crystallization of eutectic impurities in molten aluminum and the electrolytic separation of aluminum and impurities by use of a diaphragm that is permeable by a molten salt electrolyte, into which are dissolved ions containing one or more aluminum atoms, but which restricts the passage of molten aluminum and constituents such as iron and silicon. Art showing the use of fractional crystallization as a means to purify aluminum includes U.S. Pat. Nos. 3,211,547 to Jarrett et al., 3,303,019 to Jacobs and 4,221,590 to Dawless et al. Patents showing the use of a permeable diaphragm in an electrolytic cell to purify aluminum include U.S. Pat. Nos. Re. 30,330 to Das et al. and 4,214,955 and 4,214,956 to Bowman. The disclosures of these patents are incorporated here by reference.
Patent application Ser. No. 369,610 to Helling et al. shows the combination of a main melting cell and two forehearths for purifying aluminum. The forehearths are used for removing "segregation grains" and for receiving fresh anode alloy and aluminum to be refined. The forehearths are joined to the main cell by sloping channels, as seen in FIG. 1 of the publication.
In FIG. 7 of U.S. Pat. No. 2,539,743 to Johnson, an initial hearth 85 is used to selectively melt aluminum and not copper and iron impurities in the aluminum. The melted aluminum is then directed to a cell 82 having electrolytic diaphragms where the aluminum is further purified. Two separate vessels are used and the vessels are connected together by channel means, as in the Helling et al. publication.
Yet another reference showing the purification of aluminum is U.S. Pat. No. 4,222,830 to Dawless et al. The disclosure of this patent is directed to the use of an electrolytic cell to first effect purification of an aluminum charge, and then to further purify the aluminum by use of a fractional crystallization cell that is separate from the electrolytic cell. The aluminum that contains high levels of impurities obtained in the latter cell can then be returned to the electrolytic cell and be mixed with primary aluminum in that cell and hence provide savings in the inventory of impure or primary aluminum required to produce high purity aluminum.